1. Field of the Invention
This invention relates to hydraulic actuators that must operate under severe conditions. More particularly, this invention relates to aircraft hydraulic actuator cylinders that must operate under harsh conditions, must have a high degree of survivability, and be light in weight.
Both fixed wing and rotary wing aircraft typically include many different types of hydraulic actuators. Such actuators operate flight control surfaces and rotor blades, extend and retract landing gear, and open and close movable panels and doors. One particular application of a hydraulic actuator is in operating the swash plate used to change the pitch of helicopter rotor blades. As a helicopter rotor rotates, the pitch of the rotor blades changes depending on the relative location where lift is required. The pitch of a rotor blade may be characterized as the angle the blade makes with respect to the plane of rotor rotation as the blade passes through the air. For example, if an airborne helicopter is to tilt to the right, increased lift will be necessary on the left side of the helicopter. As each blade of the rotor reaches the left side of the helicopter, the pitch of the blade is changed so as to create additional lift on that side. As the blade leaves the left side of the helicopter and approaches the right side, the pitch of each blade is changed so as to reduce the amount of lift created on the right side relative to the left side. Similarly, if the helicopter is to pitch forward, increased lift will be needed from the blades as they approach the tail of the helicopter and less lift is required at the nose of the helicopter. Thus, the pitch of the helicopter blades will be changed as they leave the nose of the aircraft and approach the tail so as to increase the amount of lift created.
More complex movements of the helicopter, such as climbinq and banking simultaneously to the left or right, will require more complex combinations of changes in the pitch of the blades during a single rotation of the rotor. Obviously, the actuator controlling the pitch of each helicopter blade may be actuated several times within a single rotation of the helicopter rotor. This requires many operating cycles of the actuator during the lifetime of the helicopter, under very severe conditions. An actuator for a helicopter blade typically operates at a hydraulic pressure of 3,000 psi and an oil temperature of 275.degree. F.
In the case of military aircraft, an additional operating requirement is a degree of survivability. Military aircraft operate in an environment in which they often are the target of small arms fire from ground troops. An aircraft is said to have survivability when it has the ability to absorb such ground fire without the loss of flight-critical systems and still return to its base for repair. Survivability is a critical and highly desirable requirement for military aircraft.
2. Description of the Prior Art
Hydraulic actuators are typically constructed entirely of metal. That is, both the actuator cylinder and the piston and rod moving within the cylinder are constructed of metal. If a bullet pierces such an actuator, the actuator cylinder wall will be deformed or dented inward where the bullet enters the cylinder. A portion of the cylinder wall then interferes with the piston travel and freezes or jams the movement of the actuator at that point. Furthermore, splintering of the metal actuator cylinder can damage other systems and can also cause other moving parts to jam.
Molded plastic actuator cylinders have been investigated in the interest of reducing cost and saving weight. Plastic alone, however, will not endure the number of operating cyles required, since the actuator piston (with piston seal loads from the hydraulic operating pressure) wears away the inner surface of the plastic cylinder. The piston seals are also subjected to accelerated wear because the molded plastic cylinder does not have the stiffness of a metal cylinder. The hydraulic operating pressures on the plastic cylinder are such that, as the piston changes direction in the cylinder, the cylinder walls are forced outward momentarily. The piston seals expand with the walls, maintaining their seal. The constant expansion and contraction of the seals as the piston moves back and forth in the cylinder results in premature seal wear. The constant expansion and contraction of the walls also stresses the cylinder. A radial expansion of two to three thousandths of an inch is practically the maximum tolerable for acceptable seal life. Thus, plastic does not withstand the high stresses with low radial deflections required for long life. Plastic also can shatter when hit by ground fire.
A metal sleeve inserted into the plastic cylinder as a lining may provide increased seal life but is not a completely acceptable solution. The point of using a plastic cylinder is to save weight and cost. A metal sleeve must therefore be thin enough to make the plastic cylinder worthwhile. The metal sleeve, however, must be of such a thickness in order to be inserted into the cylinder that the problems of bending and survivability are again present. In addition, the metal sleeve is very stiff when compared with the plastic cylinder. The metal sleeve may prevent radial expansion, but will become overstressed in trying to provide a reinforcing structure for the plastic cylinder. If the metal sleeve is made thick enough to avoid overstressing, the weight and cost savings of using a plastic cylinder are decreased.
Filament winding techniques have been used for cylinders because structures made from such techniques will not bend or dent when hit by small arms fire. In such techniques, continuous high-strength fibers that are coated or impregnated with a thermoplastic epoxy resin are wound onto a mandrel, forming a composite cylinder. The fibers allow bullets to pass through cleanly without shattering the cylinder and without causing the actuator to jam or freeze. Structures made from filament winding techniques also have the required stiffness for longer seal life. Additionally, such structures can be produced at reduced cost and lighter weight when compared to metal actuator cylinders. As with plastic cylinders, however, composite cylinders alone will not provide a sufficient number of operating cycles before being worn down by the high piston and seal loads. A metal sleeve inserted into the cylinder is unacceptable due to the bending and survivability problems, as well as weight, cost, and manufacturing problems.
Other liner techniques have been tried with both molded plastic and composite cylinders. For example, metal plating techniques have been tried where a lining of electrolus nickel is plated onto the interior surface of the actuator cylinder by means of a chemical bath. Unfortunately, metal plating techniques do not achieve a good bond between the plastic or composite cylinders and the metal layer. Thus, the electrolus nickel lining does not remain bonded to the interior surface of the actuator cylinder, and typically chips off after only approximately 50,000 cycles of the actuator. A helicopter rotor blade actuator must endure many times that number of operating cycles during its lifetime, with the general requirement being upwards of three million cycles with no repair required. Therefore, there is a need for a hydraulic actuator cylinder that is durable enough to operate in a severe environment, can be produced at lower cost, is light weight, and has a high degree of survivability.